I would say 4 chins can fit in a double CN, however that doesn't give them much room to get away from each other if they want/need to. 2'x2'x2' (or 8 cubic feet) is the minimum space needed per chin, it can overlap some though, so a 2'x2'3' (the size of one CN unit) is ok for two, but can get cramped and you increase the chance of fighting.
I also personally don't advise trying to add chins to an already bonded pair, a lot can go wrong. The pair can gang up on the new chin, the pair can switch so you have a new pair and a single that use to be in a pair, and you could end up with all the chins hating each other. Most cases I've seen where you have multiple that live together successfully are related, mother/daughters, father/sons, or same sex siblings. I'm not saying it never works otherwise, but just be aware that it might not and you will then need to house the chins all separately. Or split the cage so two are on top and two on the bottom, though that doesn't seem fair to the current two that are use to having the whole cage.